1. Field of the Invention
This invention relates generally to operational amplifiers, and more particularly, to an all NPN monolithically integrable output stage for integrated operational amplifiers in which excess phase shift due to capacitive loads and second order distortions due to non-linearities in the input/output transfer function are both substantially reduced.
2. Description of the Prior Art
A large majority of commercially available operational amplifiers which possess both pull-up and pull-down output capability include PNP devices. To achieve a higher frequency response, greater output swing, reduce output stage emitter follower peaking and excess phase with capacitive loads, and simplify integrated circuit construction, it has been found desirable to provide an output stage which incorporates only NPN transistors. Such an output circuit is shown and described in U.S. Pat. No. 3,416,092. However, there are several problems associated with this circuit. First, the output voltage is not linearly related to the input voltage as a result of crossover distortion produced by the classic deadband technique. Second, when the output is sinking current from the load, the output voltage can only be pulled to within approximately 1 volt of the negative rail voltage regardless of the sink current magnitude. Third, when the output sources current to the load, the output voltage can only swing to within approximately 1.8 volts of the positive rail. Finally, no part of the output stage inherently provides any controlled current limiting capability.
Co-pending U.S. patent application, Ser. No. 244,411 filed Mar. 16, 1981, entitled "Amplifier Output Stage" and assigned to the assignee of the present invention describes an all NPN output stage wherein the crossover distortion due to deadband has been eliminated and, as a result of using only NPN transistors, the output quiescent current can be precisely defined and controlled over processing variations and temperature. A first NPN transistor sinks current from the output of the circuit when the first transistor is turned on by a varying input signal. A second transistor is coupled to the output of the circuit and sources a second current to the output when the first transistor turns off. A resistor coupled between the collector of the first transistor and the emitter of the second transistor provides a voltage drop thereacross proportional to the amount of current being sunk by the first transistor so as to control the voltage at the base of the second transistor. In this way, the second transistor is turned on and off as a first transistor is turned off and on respectively. Diode means provide a voltage level shift between the resistor and the base of the second transistor.
While eliminating distortion due to deadband, the circuit of the above cited application still suffers from a second order large signal distortion as a result of non-linearities in the transfer function due to large current variations in both the first NPN sink transistor and the second NPN source transistor. It is important to note that with the addition of a Miller compensation capacitor across the stage containing the first NPN transistor, the distortion at the collector of the first NPN transistor contains primarily the nonlinear terms associated with the first NPN transistor. In contrast, the distortion from the collector of the first transistor to the output is significantly greater due to the gain variations associated with the output current source circuitry including the second NPN transistor; i.e. the major distortion is caused by the source circuitry. Furthermore, if the output of the circuit were to become short circuited to the positive supply rail, the emitter-base junction of the second transistor may break down.